_acme-challenge.postmaster.heysender.com has no MX records.
Question and response
Question
dig postmaster.heysender.com.kinstavalidation.app. MX
Answer
The response has no answer records.
Authority
kinstavalidation.app. 1800 SOA elliott.ns.cloudflare.com. dns.cloudflare.com. 2397451090 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0